Corrosion due to increased oxidation-reduction potential in aqueous systems, such as industrial boilers and other hot water systems, is a major concern. The affinity of oxygen for alloys used in the boiler water industry is the cause of many corrosion phenomena. This corrosion is a complex process that depends not only on the amount of oxygen in the system, but also on factors such as the water chemistry and metallurgy. For example, the presence of other species in the water could turn oxygen into an aggressive corrosive force, or could render the metallurgy passivated. Other important factors are temperature, pressure, fluid velocities, and operational practices. While oxygen might be the primary or essential component in the corrosion process, it might not be the only one.
The conventional means for reducing oxygen corrosion in hot water systems is to remove most of the molecular dissolved oxygen by mechanical and chemical means. The vast majority of the dissolved oxygen is reduced into the parts per billion range by the use of mechanical deaeration. The water is typically heated to above boiling temperature in a vented vessel leading to a decrease in dissolved oxygen solubility as the temperature increases. Flow dynamics and operational issues particular to deaerators leave parts per billion of dissolved oxygen in the water. Oxygen scavengers are chemicals used to reproducibly reduce dissolved oxygen values to low and constant values. Many of these scavengers also function as passivating corrosion inhibitors. Deaerators do not always work perfectly; if they did, a pure scavenger might never be needed, although a chemistry that enhances metal passivation would be a positive addition. In some cases, the oxygen scavenger is added as an insurance policy against the possibility that the deaerator might malfunction. The scavenger can also be added to combat air leakage into the system.
Traditionally, the amount of oxygen scavenger fed to boiler feedwater has been based on the amount of dissolved oxygen in the feedwater plus some excess amount of scavenger. The amount of excess scavenger fed is based on the desired residual scavenger concentration in the boiler feedwater or boiler water itself, which is a function of the excess concentration of scavenger and boiler cycles. There are several problems with this feed control scheme. The first is that there is no active control of the scavenger feed rate. High oxygen conditions could exist for long periods before a decrease in residual scavenger occurs and corrective action is taken.
A second issue is that the presence of residual scavenger in the boiler water simply does not mean that the system is being treated satisfactorily. Depending on the conditions (e.g., low temperature or short residence time) it is possible to have both high oxygen concentrations and sufficient scavenger in the feedwater at the same time. When this oxygen rich feedwater reaches the boiler, oxygen is flashed off with the steam leaving the unreacted scavenger in the boiler water. In an extreme case, the result may be an unacceptably high dissolved oxygen level in the pre-boiler and condensate systems while having expected residual concentrations of oxygen scavenger in the boiler itself.
In certain high-pressure boilers (once through) that use ultra-high purity water, a different approach has been taken. No oxygen scavengers are used in such systems. Instead, small amounts of molecular oxygen are deliberately added to the feedwater. Oxygen (i.e., the oxidant) acts as the passivating agent for carbon steel under carefully controlled conditions of boiler water chemistry. Oxygen concentrations used are much less than the air saturated (8 ppm dissolved oxygen) values, thus some deaeration is used. It is often easier to deaerate to some extent prior to adding a controlled amount of oxygen.
Corrosion in industrial boiler systems typically occurs at operating (i.e., elevated) temperature and pressure. The most effective and accurate operational and control data is based on measurements taken under actual operating conditions. Gathering such data, which is indicative of corrosion stress on the system, at boiler feedwater temperature and pressure is difficult and seldom done. Traditionally, oxidation-reduction potential has been measured at room temperature and pressure in a sample taken from the system. Such room temperature measurements and other traditional measurements, such as dissolved oxygen, metallurgy-specific corrosion rate, or scavenger residual measurements, are incapable of detecting many corrosion events and stresses.
There thus exists an ongoing need for effectively measuring and monitoring the corrosion potential for the various metallurgies used in aqueous systems. Such monitoring would enable proactive adjustment of, for example, in hot water systems, feedwater chemistry (such as oxygen, oxygen scavengers, reducing agents, and oxidizing agents) rather than reactive adjustments after corrosion has already occurred. Continuous, real-time optimization of feedwater chemistry including an oxygen scavenger/passivation program would prevent corrosion problems that lead to lost steam production, downtime, reduced asset life, and higher operating costs.